(A) Field of the Invention
The present invention relates to a surface mountable laminated circuit protection device and method of making the same, in particular, to a surface mountable laminated circuit protection device having positive temperature coefficient (PTC) characteristics and the method of making the same.
(B) Description of Related Art
PTC devices are already widely used in various fields, such as temperature detection, security control, temperature compensation, and so on. In the past, the thermistor device was generally made from ceramic material. However, the ceramic material was formed at high temperatures, in most cases more than 900xc2x0 C., thus rendering the energy consumption enormous, and making the production process very complex. Subsequently, a thermistor device made from a polymeric substrate was developed. As the temperature for manufacturing a thermistor device made from a polymeric substrate is under 300xc2x0 C., its molding and manufacturing is easier, energy consumption is less, the production process is simpler, and production cost is lower. As a result, this kind of thermistor device has become more and more popular.
U.S. Pat. No. 5,852,397 discloses a polymeric composite material filled with a conductive filler to form a PTC circuit protection device. The polymeric composite material filled with a conductive filler having PTC characteristics is under a low resistance status at room temperature; when the current flowing through the polymeric composite material is too large, the temperature of the polymeric composite material reaches a certain switching temperature (Ts), and the resistance of the polymeric composite material filled with a conductive filler increases rapidly to prevent important devices in the circuit from being burnt down; this characteristic can be applied to the design of over-current protection devices and temperature switch devices. This phenomenon is due to the fact that the conductive filler particles in the polymeric composite material filled with the conductive filler are at continuous and conducting status at room temperature. When the temperature rises to above Ts, the volume of the resin in the polymeric composite material expands to an extent that makes the conductive filler particles in the polymeric composite material break down from a continuous status to a discontinuous status; the resistance of the PTC circuit protection device thus rises rapidly to break the current to achieve the objectives of over-current protection and temperature control switch. Various different materials are used as conductive filler, with the most common being carbon black.
U.S. Pat. No. 6,023,403 discloses a PTC laminated structure of a conductive composite material device that has a top metal foil layer, a bottom metal foil layer and a middle layer having PTC characteristics. Combined with a side-conducting mechanism and insulating material, it conducts the top and bottom metal electrodes of the conductive composite material having PTC characteristics to another side to form a surface mountable circuit protection device.
R.O.C. Patent Published No. 419,678 discloses a PTC laminated structure of a conductive composite material device that has a top metal foil layer, a bottom metal foil layer and a middle layer having PTC characteristics. It combines with a plated through hole conducting mechanism and applies an etching process to form a discontinuous cross-section on the top and bottom metal electrode layers for conducting the top and bottom metal electrodes of the conductive composite material having PTC characteristics to the same side, then applies more than two similar top and bottom metal electrodes, conducting PTC laminated structure, and insulating layer to form a parallel connected surface mountable circuit protection device.
Prior art mainly utilizes metal foil and conductive composite material elements having PTC characteristics to form a PTC laminated structure using the thermal laminating process, then performing electroplating process, etching process, plating through hole and lateral end point silver process. The mechanical strength of a PTC laminated structure formed by metal foil/conductive composite material device having PTC characteristics/metal foil is inadequate; it tends to wrap and become deformed during the processes mentioned on. When it comes to laminating with other PTC laminated structure, strengthened insulating material or metal electrode by thermal laminating process after circuits have been made, there is a problem with the accuracy of location correspondence between upper and lower layers.
Furthermore, prior art already uses carbon black to directly wedge to metal nodular protrusions; the geometric shapes of carbon black and of metal nodular protrusions are different, so the contact density is not very well. Meanwhile, the mobility of resin on the surface of carbon black is not good between carbon black and metal; sometimes it just adheres to the surface of the metal, thus increasing the resistance of the interface and affecting its functioning.
Moreover, the production method of prior art involves laminating metal foil and conductive composite material element having PTC characteristics by thermal laminating process first, and then proceeding with plating through hole process or lateral end-point silver process of passive device to conducting top and bottom metal electrodes, thus forming a circuit protection device. The conducting method between the internal electrodes of the circuit protection device is limited by this fabrication method.
An object of the present invention is to provide a process for manufacturing a surface mountable laminated circuit protection device, which utilizes a well-developed process used in printed circuit board (PCB) production during the process of the present invention to make the manufacturing of circuit protection devices easier.
Another object of the present invention is to provide a surface mountable laminated circuit protection device with better structural strength and dimensional stability.
Yet another object of the present invention is to provide a surface mountable laminated circuit protection device possessing symmetric structure, which can be processed on both sides at the same time to make manufacturing more convenient.
Still another object of the present invention is to provide a surface mountable laminated circuit protection device which forms a fine contact between the metal and the conductive composite material to reduce the interfacial resistance between them and improve the functioning of the circuit protection device.
To achieve the objects described above, the present invention provides a surface mountable laminated circuit protection device comprising a first metal layer including a first unit and a second unit. A first insulating layer is disposed on the first metal layer, and a second metal layer is disposed on the first insulating layer. There is also a composite electroplated layer containing carbon black disposed on the second metal layer, and a first conductivity composite material layer having positive temperature coefficient (PTC) characteristics disposed on the composite electroplated layer containing carbon black; it is jointed to the second metal layer by means of a composite electroplated layer containing carbon black. Above the first conductivity composite material layer having PTC characteristics there is a third metal layer. Furthermore, there is a first conducting mechanism for conducting the second metal layer and the second unit of the first metal layer to each other, and a second conducting mechanism for conducting the third metal layer and the first unit of the first metal layer to each other.
Moreover, the present invention provides a method of making a surface mountable laminated circuit protection device, which uses the following steps: First, provide a double-sided foil clad substrate. The double-sided foil clad substrate comprising a first metal layer, a first insulating layer disposed on the first metal layer, and a second metal layer disposed on the first insulating layer. A plated through hole penetrates through the insulating layer for conducting the first metal layer and the second metal layer to each other; the first metal layer is further divided into a first unit and a second unit. A composite electroplating process with carbon black is proceeded to the second metal layer, it is to form a composite electroplated layer containing carbon black and metal on the surface of the second metal layer. A first conductivity composite material having PTC characteristics and a metal foil are then laminated in sequence onto the surface of the second metal layer using the thermal laminating process to join the first conductivity composite material having PTC characteristics and the second metal layer; the metal foil is further joined with the first conductivity composite material having PTC characteristics, thus forming a multi-layer laminated circuit structure, and the metal foil itself is taken as a third metal layer. An isolation step is proceeded to the third metal layer to make the third metal layer forming a third unit and a fourth unit. There is a first conducting mechanism set for conducting the third unit of the third metal layer and the first unit of the first metal layer to each other, and also a second conducting mechanism set for conducting the fourth unit of the third metal layer and the second unit of the first metal layer to each other.
In accordance with the description given on, the method of the present invention utilizes a double-sided metal foil clad substrate; it can directly fit in with the current well-developed process of printed circuit board to make the manufacturing of the laminated circuit protection device easier. Furthermore, the surface mountable laminated circuit protection device provided by the present invention uses a strengthened insulating layer to give the device better structural strength and better dimensional stability. In addition, a composite electroplated layer containing carbon black is formed on the metal layer; it can be tightly integrated with the first conductivity composite material having PTC characteristics, thus forming a fine joint for better functioning of the circuit protection device.